The present invention relates to lithiated manganese dioxides, to methods of making such materials and to electrochemical cells incorporating the materials as cathode-active materials.
Electrochemical cells useful as electrical storage batteries usually incorporate a metallic anode and a cathode including an active material which can take up ions of the metal. An electrolyte incorporating ions of the metal is disposed in contact with the anode and the cathode. During discharge of the cell, metal ions leave the anode, enter the electrolyte and are taken up in the active material of the cathode, resulting in release of the electrical energy. Provided that the reaction between the metal ions and the cathode active material is reversible, the process can be reversed by applying electrical energy to the cell. If such a reversible cathode-active material is provided in a cell having the appropriate physical configuration and an appropriate electrolyte, the cell can be recharged and reused. Rechargeable cells are commonly referred to in the battery art as "secondary" cells.
It has long been known that useful cells can be made with an anode of a light alkaline metal such as sodium, potassium and, particularly, lithium, and a cathode-active material which is a sulfide or oxide of a transition metal, i.e., a metal capable of assuming plural different valence states. Dampier, "The Cathodic Behavior of CuS, MoO.sub.3, and MnO.sub.2 in Lithium Cells", J.Electrochem. Soc., Vol. 121, No. 5, pp. 656-660 (1974) teaches that a cell incorporating a lithium anode and a manganese dioxide cathode-active material can be used as an electrical power source. The same reference further teaches that a lithium and manganese dioxide cell can serve as a secondary battery. It is common knowledge that water reacts violently with the reactive alkaline metals, and hence is regarded as undesirable in any component of a cell having an alkaline metal anode.
West German OLS No. 2 058 910 teaches that the ion exchange capability of manganese dioxide can be enhanced by pretreating the manganese dioxide with lithium ions, as in an aqueous solution of LiOH so as to saturate the manganese dioxide with Li and then baking the saturated MnO.sub.2 at a temperature of about 450.degree.-500.degree. C. for about 3-6 hours. The amount of Li used in the saturation step is said to be about 5.75-6.10 milliequivalents per gram, i.e., about 0.50 to about 0.53 moles Li per mole MnO.sub.2. Acta Chemica Sinica, Vol. 39, No. 8, pp. 711-716 discloses a similar procedure applied to another form of MnO.sub.2 referred to as electrolytic MnO.sub.2, and likewise suggests that the procedure results in an ion exchange material having enhanced ion exchange capabilities and good reversibility. The formula of the ion exchange material is given as "LiMn.sub.2 O.sub.4 ". Although these ion-exchange teachings do not specifically refer to use of MnO.sub.2 as a cathode active material in an electrochemical cell, it has long been known that the ion exchange properties of a material such as manganese dioxide are closely related to its performance as a cathode active material in a cell. This relationship is disclosed, for example, in Kozawa, "On an Ion-Exchange Property of Manganese Dioxide", J. Electrochem Soc., Vol. 106, pp. 552-556 (1959).
In consonance with these teachings, Hitachi Maxell KK, Japanese patent publication 59-31182 (1984) teaches a non-aqueous electrolyte primary battery having a light metal anode and a cathode-active material of manganese dioxide which has been soaked in a solution of a light metal ion such as a lithium hydroxide solution and then heat-treated at about 200.degree. to about 400.degree. C., preferably at about 300.degree. C. The '182 publication notes that manganese dioxide generally contains a substantial amount of adhering or bound water, and that heat treatment at elevated temperatures serves to drive off this water and hence improves the discharge performance and shelf stability of a non-aqueous Li/MnO.sub.2 cell. The '182 publication states that manganese dioxide subjected to such heat treatment may undergo undesirable changes upon heat treatment but that the presence of metal ions in the MnO.sub.2 during heat treatment controls these changes. The '182 publication further states that the beneficial effect of the metal component is particularly pronounced in the case of electrolytic gamma phase manganese dioxide. The Hitachi '182 disclosure does not specifically mention the use of the treated MnO.sub.2 as a cathode active material in a secondary battery.
Other, more recent publications dealing with a cathode-active material containing both lithium and manganese dioxide have been directed towards secondary battery applications. Matsushita, Japanese patent application laid open (Kokai) 62-20250 (1987) discloses preparation of a lithiated manganese dioxide of the general formula Li.sub.y MnO.sub.2 by a route which does not involve lithium treatment of existing MnO.sub.2. Instead, the '250 publication proposes synthesis of a lithium-potassium permanganate of the general formula (1-x)K.sup.. xLi.sup.. MnO.sub.4, followed by thermal decomposition of the permanganate. The '250 publication teaches that decomposition products having stoichiometric formulas between Li.sub.0.3 MnO.sub.2 and Li.sub.0.8 MnO.sub.2 provide good resistance to loss of cycling capacity upon repeated charge and discharge cycles in the battery. Sanyo, Japanese Kokai 62-108455 (1987) describes a secondary battery having a cathode-active material including lithiumdoped gamma phase electrolytic manganese dioxide prepared according to the same general scheme as employed in the Hitachi '182 publication. Thus, Sanyo '455 teaches immersion of electrolytic manganese dioxide particles in one molar lithium hydroxide solution and heating to the boiling point of the solution, followed by repeated additions of more lithium hydroxide solution and repeated boiling. Following this immersion and boiling treatment, the MnO.sub.2 particles are washed in water and then heat treated at between 350.degree. to about 430.degree. C. for two hours or more. The resulting cathode-active material is characterized as having the desirable gamma type crystalline structure, as being substantially free of moisture and as providing good cycling characteristics. Sanyo, Japanese Kokai 61-16473 (1987) suggests conducting aqueous LiOH treatment of MnO.sub.2 at a superatmospheric pressure and at a temperature above 100.degree. C., typically, 180.degree. C. Toshiba, Japanese Kokai 62-126556 (1987) teaches treatment of the particulate MnO.sub.2 in excess LiOH solution with stirring at about 60.degree. C. followed by filtering and drying at 100.degree. C. with subsequent heat treatment at 350.degree. C. for 24 hours. Toshiba '556 teaches that the discharge capacity of a battery (apparently a primary battery) made using the so-treated MnO.sub. 2 can be optimized by selecting an LiOH solution of 0.05 to 0.02 molar concentration for use in the liquid phase treatment step. Toshiba '556 attributes this effect to the absence of sufficient lithium ions in solution below 0.05 molar and formation of a "colloidal" surface on the MnO.sub.2 particles if the lithium solution concentration exceeds 0.2 molar.
Thus, there has been considerable effort by prominent industrial companies in the battery field directed towards development of cathode-active materials including both Li and MnO.sub.2. There has been a significant commercial incentive for these efforts. Both lithium and manganese dioxide are relatively inexpensive, readily obtainable materials, offering the promise of a useful, potent battery at low cost.
Despite all of this effort, there have been unmet needs heretofore for further improvements in processes for making cathode-active materials, in cathode-active materials and in cells incorporating these materials.